Electronic devices of all kinds employ use of phase-locked loops (sometimes called phase lock loops) for manipulating electronic signals. A phase-locked loop (PLL) is a control system that generates an output signal having a phase and frequency that is related to the phase and frequency of an input signal (often called a reference signal). The circuit compares the phase of the input signal with the phase of the signal at the output and adjusts the frequency of an oscillator to keep the phase of the generated output signal matched to the phase of the input signal. By keeping the phases of these signals locked to each other, one can be assured that the frequency of the output signal also matches the frequency (or an integer multiple thereof) of the input signal.
Keeping the input and output signals locked in to the same frequency allows for higher and higher speed signal processing and communication to occur. PLL circuits are widely employed in radio, telecommunications, computers, and other electronic applications. In various applications, a PLL circuit may be used to recover a signal from a noisy communication channel, generate stable frequencies at a multiple of an input frequency (frequency synthesis), or distribute clock timing pulses in digital logic designs such as microprocessors. Since a single integrated circuit can provide a complete PLL circuit, the technique is widely used in modern electronic devices, with output frequencies from a fraction of a hertz up to many gigahertz.
As PLL circuits are called upon to be employed in devices having faster speed requirements and having lower power supply voltages, electronic noise, such as thermal noise, and device size become larger factors in the design of the PLL.